Our application proposes to examine the mechanisms by which bone formation is impaired in pediatric Crohn disease. Crohn disease is a chronic inflammatory disorder of the gastrointestinal tract. Bone mineral density is decreased in about 50% of patients at the time of diagnosis, before exposure to corticosteroids. Our studies have shown that biochemical markers of bone turnover are significantly decreased in newly diagnosed children with Crohn disease. We hypothesize that bone formation is reduced in these children, We believe that this is secondary to the inflammatory response present in this disorder. In Crohn disease, CD4+Thl cells are characteristically activated and secrete interferon (IFN)-gamma, among other products. We have shown that IFN-gamma plays a critical role in the inhibition of osteoblast differentiation and function in vitro by activated T cells. We hypothesize that IFN-g blocks the signal transduction of bone morphogenetic protein (BMP)-2, a critical factor in early osteoblast development. Our specific aims are: 1. To examine calcium (Ca) absorption, excretion, and incorporation into bone in newly diagnosed children with Crohn disease using stable Ca isotopes. Concurrently we will determine bone mineral density, measure biochemical markers of osteoblast activity and bone matrix degradation and then correlate these parameters with clinical variables such as the pediatric Crohn disease activity index and IFN-gamma synthesis by activated T cells from the peripheral circulation and inflamed colon. 2. To examine the effects of IFN-gamma on components of the BMP-2 signaling cascade in cells of the osteoblast lineage obtained from mice with chronic colonic inflammation (Interleukin-10 knockout mice) and in a mouse bone marrow stromal cell line (ST-2 cells). Our studies will involve a unique collaboration of experts in pediatric gastroenterology, bone cell biology, signal transduction, and human nutrition and will provide important insight into the mechanisms of bone loss in Crohn disease and other childhood diseases characterized by abnormal T cell activation.